1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as printers, plotters, and copiers, include an optical scanning device that deflects light beams from an light source by a light deflecting unit such as rotating polygon mirror, focuses the deflected light beams on a target surface through a scanning and focusing optical system such as fθ lens to form a light spot on the surface, and scans the surface with this light spot.
In the process of forming an image, an image is written by optical scanning. Quality of the image formed by the process is influenced by quality of the optical scanning. The quality of optical scanning depends on scanning characteristics in the main scanning direction and in the sub-scanning direction in the optical scanning device.
As one scanning characteristic in the scanning direction may be cited velocity uniformity of optical scanning. For example, when a rotating polygon mirror is used as a light deflecting unit, since deflection of light flux is conducted at uniform velocity, a scanning and focusing optical system having fθ characteristic is used for realizing uniform velocity of optical scanning. However, it is not easy to realize complete fθ characteristic because of relations with other characteristics required for the scanning and focusing optical system. Therefore, in practice, optical scanning is not conducted at perfectly uniform velocity, and velocity uniformity as a scanning characteristic is accompanied with deviation from ideal scanning at uniform velocity.
Examples of scanning characteristics in the sub-scanning direction include curve and inclination of scanning line. A scanning line is a locus of movement of light spot on a target surface, and is ideally a straight line, and an optical scanning device is designed so that the scanning line is straight. However, in practice, curve usually occurs in the scanning line due to processing error, assembling error or the like. When a focusing mirror is used as a scanning and focusing optical system and a certain angle is made in the sub-scanning direction of deflected light flux between incident direction and reflection direction to/from an focusing mirror of the deflected light flux, in principle, scanning line curves or inclines. Even when the scanning and focusing optical system is arranged as a lens system, scanning-line curve is inevitable in a multi-beam scanning system which optically scans a target surface with a plurality of light spots separated in the sub-scanning direction.
Scanning-line inclination indicates that the scanning line does not appropriately cross the sub-scanning direction at a right angle, and is one type of scanning-line curve. In the following, the term “scanning-line curve” includes scanning-line inclination unless otherwise specified.
Imperfect velocity uniformity of optical scanning causes distortion in the main scanning direction in the formed image, and scanning-line curve causes distortion in the sub-scanning direction in the resultant image. At this time, when an image is written and formed by a monochromatic, single optical scanning device, distortion “that is recognizable at one view” does not occur in the formed image so far as scanning-line curve and imperfectness of velocity uniformity (deviation from ideal uniform velocity scanning) are suppressed to some extent. However, it is still desired to control such image distortion as small as possible.
In color copiers and the like, images of three color components: magenta, cyan, and yellow, or four color components: magenta, cyan, yellow, and black, are superimposed one upon another to form a color image.
A tandem image forming system is used for forming color images, in which an image of each color component is formed on a photoconductor using an optical scanning device provided for each color component. In such an image forming system, when degrees of scanning line curve or inclination mutually differ between different optical scanning devices, color misregistration or color shift occurs in the color image even if scanning-line curve is corrected at least for each optical scanning device, and impairs image quality of the color image. For example, color shade may not be desired one in a color image due to color misregistration.
To prevent such color misregistration, for example, Japanese Patent Application Laid-Open No. 2002-131674 (Claim 1 and FIG. 3) has proposed a structure that corrects scanning-line curve by implementing a supporting part on either side which sandwiches an optical axis of long lens in the sub-scanning direction by an adjusting part using an adjusting screw capable of moving in the optical-axis direction of the long lens, and rotating and adjusting the long lens in the section perpendicular to the deflecting and scanning direction through degree of fastening of the adjusting screw.
Japanese Patent Application Laid-Open No. 2005-241753 (Claim 1 and FIG. 6) has proposed an optical scanning device which effectively suppresses deformation due to temperature change of focusing elements made of resin involved in an scanning and focusing optical system and is capable of accurately correcting any one of scanning-line curve and velocity uniformity or both.
However, the structure disclosed in Japanese Patent Application Laid-Open No. 2002-131674 cannot cope with environmental variation that affects material of the lens used in a focusing optical system.
In brief, with the recent intention to improvement of scanning characteristics, a specific surface represented by aspheric surface is generally employed in a focusing optical system of optical scanning device, and focusing optical systems fabricated from resin materials that are facilitate formation of such specific surface and low in costs are often used. However, optical characteristic of a focusing optical system made of resin is easy to change under influence of change in temperature or humidity, and such change in optical characteristic also changes the degree of scanning-line curve or velocity uniformity. Therefore, when several tens of color images are sequentially formed by the image forming apparatus, the internal temperature increases, which changes optical characteristic of focusing optical system. This also changes degree of scanning-line curve for writing in each optical writing device, and gradually changes velocity uniformity, resulting in that a color image obtained in early stage and a color image obtained in final stage may have completely different color shades due to color misregistration.
A scanning and focusing lens such as fθ lens which is representative of a scanning optical system is generally formed as a strip-shape lens that is long in the main scanning direction by cutting a lens unnecessary part (part where deflected light flux does not enter) in the sub-scanning direction. When the scanning and focusing lens is made up of a plurality of lenses, the farther the disposed position is from the light deflecting unit, the larger the length of the lens in the main scanning direction, and a long lens having length of 10 and several centimeters to 20 centimeters or longer is required. Such a long lens is generally formed by resin molding. However, when temperature distribution in the lens becomes nonuniform due to change in external temperature, warpage occurs and the lens becomes arcuate in the sub-scanning direction. Such warpage of long lens may cause scanning-line curve as described above, and scanning-line curve enormously occurs when the warpage is significant. Such phenomenon occurs even when initial adjustment is conducted using the structure shown in Japanese Patent Application Laid-Open No. 2002-131674. In the structure, no countermeasure against scanning-line inclination which is a cause of defects such as color misregistration as well as scanning-line curve is taken. Further, the structure has such a drawback that positioning accuracy is difficult to be ensured because positioning of lens in the optical-axis direction changes depending on the fastening degree of screw.
On the other hand, in Japanese Patent Application Laid-Open No. 2005-241753, change in shape (warpage, etc.) of focusing element made of resin caused by temperature change is suppressed by surrounding the focusing element made of resin by a member of relatively high rigidity. However, with this structure, the number of parts increases and the size of module increases, so that a problem arises in mechanical layout inside the apparatus.
Also, for example, heat generated by driving of a rotating polygon mirror is transmitted to an optical element group together with the air flow of flight generated by rotation of the polygon mirror that is usually rotated and driven at high speed. As a result, temperature of the optical element group rises, and optical characteristic thereof is changed. Particularly, a problem arises in temperature of scanning lens that is, for example, a fθ lens at closest position of the polygon mirror contained in the optical element group.
That is, when the light deflector containing the polygon mirror and the scanning lens are located in the same space, air flow of high temperature generated by high speed rotation of the polygon mirror directly contacts the scanning lens to raise the temperature of the scanning lens, and at this time, since temperature rises in such a state that the scanning lens has temperature distribution in the main scanning direction and the sub-scanning direction under influence of distance from the light deflector that is source of heat generation and orientation of air flow corresponding to the rotation direction of the polygon mirror rather than uniformly raising the temperature of the scanning lens, shape accuracy and refractive index of the scanning lens change, a spot position of laser beam shifts, and positional deviation arises, and in the image forming apparatus, image quality is deteriorated, for example, due to color misregistration.
This problem is particularly significant for the case of a lens made of resin having large coefficient of thermal expansion and low heat conductivity. This is important issue in the recent circumstances that an optical element having specific surface represented by aspheric surface is generally employed in a focusing optical system of an optical scanning device with the intention to improvement in scanning characteristic, and a focusing optical system using optical elements made of resin as an optical element group is often employed to easily form an optical element having such specific surface with low costs.
In particular, a scanning and focusing lens such as fθ lens that is representative of a scanning lens is generally formed as a strip-shape lens that is long in the main scanning direction by cutting a lens unnecessary part, that is, the part other than the part where deflected light flux enters, in the sub-scanning direction, and has large lens length in the main scanning direction. Therefore, when it is made of resin as described above, warpage arises and the lens assumes a shape which is arcuate in the sub-scanning direction, namely a shape which is arched when the lens is viewed from the optical-axis direction, when the temperature distribution inside the lens is nonuniform, which is very problematic.
As described above, optical characteristic of the scanning lens molded of a resin material is easy to change under influence of temperature, and such change in optical characteristic also changes the degree of scanning-line curve and velocity uniformity. Therefore, in an image forming apparatus, for example, when several tens of color images are formed continuously, internal temperature increases due to the continuous operation, and optical characteristic of focusing optical system changes, and degree of scanning-line curve for writing in each optical writing device (optical scanning device) and velocity uniformity gradually change, resulting in that a color image obtained in early stage and a color image obtained in final stage may have completely different color shades due to color misregistration. This color misregistration is peculiar and significant in an image forming apparatus.
If distance between the outer circumference of motor housing of light deflector and scanning lens differs in the main scanning direction, difference arises by radiation and conduction of heat, so that temperature distribution in the main scanning direction is likely to occur in the scanning lens. Further, in the case of an optical scanning device in which scanning lenses are disposed symmetrically with respect to the light deflector, temperature distribution of each scanning lens is opposite to each other in the main scanning direction, so that difference in changes of shape accuracy and refractive index of scanning lenses symmetrically disposed tends to increase, leading further increase in color misregistration as described above.
On the other hand, temperature rise due to heat generation of light deflector induces slight movement of polygon mirror and other flange to which rotor magnet is secured, rotating parts such as shaft, and high-load polygon mirror having especially large mass proportion, and changes the balance of rotation of polygon mirror and the like, thereby causing vibration. In other words, when coefficient of thermal expansion differs between rotating parts such as polygon mirror, or when strict management and inspection of tolerance of parts or securing method are failed although coefficient of thermal expansion matches between such parts, slight movement occurs due to change in balance of such rotating members during high temperature and high speed rotation, which may eventually increase vibration.
The problems of such positional difference, color misregistration and vibration clearly arise in the present circumstance in which it is requested to rotate a light deflector at high speed of not less than 25,000 rpm with high accuracy in a tandem image forming apparatus having for example, four photosensitive drums or photoconductors arranged in a convey direction of recording paper, for obtaining a color image by making latent images through simultaneous exposure by a plurality of scanning optical systems corresponding to the respective photosensitive drums, visualizing these latent images at developers using different colors of developing agents such as yellow, magenta, cyan and black, and sequentially superimposing these visualized images on the same recording paper, followed by transferring, for realizing high speed printing and high quality in an image forming apparatus.
For responding to recent demand for decrease in diameter of laser beam emitted to a surface of an image carrier, and increase in image quality, it is necessary to increase diameter of laser beam that enters the polygon mirror, so that diameter of inscribed circle of polygon mirror, and surface width in the main scanning direction and the sub-scanning direction tend to be relatively large and the load tends to be higher. However, increase of load results in increase in amount of heat generation due to increase in consumed power, and make the problem of positional deviation, color misregistration and vibration significant.
In an image forming apparatus having, as writing units of different colors: yellow, magenta, cyan, and black, optical scanning devices corresponding to the respective colors as with a tandem image forming apparatus, there is a problem that shape and refractive index of each scanning lens change, optical characteristic changes, deviation of spot position of laser beam on the surface of an image carrier, and scanning-line curve occur, relative positions of scanning lines of respective colors differ from each other, color misregistration significantly occurs, and quality of color image is significantly deteriorated, particularly caused by temperature change due to influence of heat generation at the light deflector in each optical scanning device.
Japanese Patent Application Laid-Open No. 2001-4948 discloses a multi-beam light source scanning apparatus that includes a plurality of light sources, a polygon mirror, and an optical system made up of a plurality of optical members for converging each laser beam reflected by the polygon mirror into a respective one of a plurality of objects to be irradiated. At least one optical member of the optical members is adapted to allow passage of all of the laser beams, and the optical member through which all of the laser beams pass is realized by a single member made of a single material.
In such an apparatus, even when an optical characteristic of the optical member through which all of the laser beams corresponding to the respective colors pass is changed due to temperature change or the like, all the laser beams are equally influenced by the change of the optical characteristic of the optical member, so that it is possible to prevent positional deviation in the scanning direction of laser beam from occurring between laser beams, for example. Therefore, when this multi-beam light source apparatus is applied to a color printer or color copier, positional deviation in the main scanning direction does not occur between laser beams that scan on the respective photosensitive drums provided in correspondence with each colors, and color misregistration in an image printed onto recording paper by the photosensitive drums can be prevented. Further, since the optical member through which all of the laser beams pass is realized by a single member made of a single material, an effect that the structure is simplified can be obtained.
However, when the light deflector and the optical member, particularly the scanning lens located closest to the light deflector is provided in the same space of the optical housing, high temperature air from the light deflector directly contacts the scanning lens, so that heat generation at the light deflector easily transmits to the optical member together with air flow accompanying high speed rotation, and temperature of the optical member rises. Further, since heat transmits also from the contact face of the optical housing, temperature distribution is not actually uniform both in the horizontal and sub-scanning directions in the scanning lens and some distribution exist. Therefore, the color misregistration occurs.
Japanese patent No. 3686644 discloses a technology for suppressing positional deviation of light spot in the main scanning direction on photoconductor. Even when temperature distribution in the main scanning direction arises inside the scanning lens L1, and optical characteristic changes, the influence exerts equally to all light beams because the light beams traveling toward different photoconductors commonly pass through the scanning lens L1, by using an apparatus employing such a structure that the scanning lens L1 that is located closer to the deflector of two scanning lenses is shared by light beams traveling toward different photoconductors.
However, even when such a structure is employed, temperature distribution actually arises in the sub-scanning direction of the scanning lens L1, so that it is often the case that influence of change in optical characteristic of the scanning lens L1 generally differs between optical beams which pass different positions in the sub-scanning direction. Therefore, color misregistration as described above sometimes arises.
Japanese Patent Application Laid-Open No. 2001-228416 discloses a technology in which a driving part other than an optical scanning device disposed either side generates heat by startup of an image forming apparatus; even when an optical housing of the optical scanning device expands in the sub-scanning direction due to the heat generation, the opposite side of the driving part equally expands in the sub-scanning direction due to heat generation of driver of a polygon scanner control circuit. Thus, color misregistration of image can be prevented by reducing difference between extension in the sub-scanning direction of one side of the main scanning direction, and extension of the other side to allow uniform expansion in the sub-scanning direction, and by keeping the scanning lines on the respective photoconductors parallel with each other.
However, in such a structure, since heat generation of controlling unit is used, temperature rise of the entire system is large in a polygon scanner rotating at a speed of 25,000 rpm that realizes high speed image formation, and in the case of scanning lens made of plastic or the like, change in refractive index is large, and positional deviation of zoom spot occurs. Therefore, color misregistration as described above may occur.
Japanese Patent Application Laid-Open No. 2005-234506 proposes an optical scanning device mounted in an image forming apparatus such as copier, facsimile machine and printer that reduces temperature rise of optical element and temperature deviation, vibration of deflecting member, and positional deviation of beam with high accuracy, as well as an image forming apparatus having the optical scanning device. However, this conventional technology does not take account of difference in “visual sensitivity” of toner color, and therefore, is incapable of reducing color misregistration (inevitably occurring with temperature change) exerted on “visual impression” to color image quality.
As described above, according to the conventional technologies, although any one of temperature change and temperature distribution or both of an optical scanning device, or an optical housing or optical elements housed therein can be reduced, but cannot be totally eliminated. Therefore, when scanning-line curve caused by temperature change or temperature distribution occurs, color misregistration is perceptible when visual sensitivity of color component corresponding thereto is high, and deterioration of color image is sensually and greatly recognized.